A quote from Air Publication 1979A, Cadets' Handbook of Elementary Flying Training !st Edition April 1943 - issued by command of the Air Council;
Page 85 chapter 14 entitled SIDESLIPPING.
"General Principles.
The manoeuvre known as sideslipping may be roughly described as making the aircraft descend through the air broadside on, while gliding. The direction in which the aircraft travels is at an angle to the direction in which the nose is pointing. We can sideslip in a turn as well as flying straight.
The advantage of the sideslip is that it permits us to increase our rate of descent, that is, to lose height more quickly, without increasing our forward speed.
By varying the amount of slip, we can vary the rate of descent......the degree to which any aircraft can be sidelipped depends on the extent to which the rudder is capable of overcoming the "weathercocking" of the aircraft, provided by the air pressure on the fin and keel surfaces This imposes a limit to the amount of bank which can usefully be applied and thus to the amount of sideslip....."
There is more - but by now you will have got the picture.
Reference to another old RAF manual of the World War 2 era, also has advice on sideslipping - but no trace can be found of the term "forward slipping".
Publication 129, the Royal Air Force Flying Training Manual - Part 1 Landplanes - dated November 1937 and reprinted June 1938, has this to say at Chapter 3 Basic Flying -paragraph 116 under the heading SIDESLIPPING.
"A sideslip means a state of equilibrium in which the aeroplane is travelling in a direction at an angle to that in which the nose is pointing. The unintentional slip is bad flying, but the intentional sideslip is a valuable manoeuvre to increase the rate of descent in a glide without gaining speed"....etc etc
So there you are. No such animal as a forward slip - not in those far gone days, anyway.